The present invention relates generally to the distillation of liquids, such as water and aqueous solutions, and in particular to a method of effecting such distillation and to an apparatus for carrying out the method.
Distillation equipment for distilling water, for instance for producing drinking water from seawater, is well known and is for instance widely used in shipboard applications. Such equipment is either operated with waste steam of the ship's turbine or with waste heat of ship's diesel engines which serve to drive the ship. This type of equipment is in form of small installations which frequently have only a single evaporation stage. They are simple to operate and can be controlled without any complications at all.
However, it is well known that distillation equipment operates completely without problems only under certain specific operating conditions. These depend upon the temperature of the heat exchange medium, the working pressure and other circumstances, which in turn determine the speed of evaporation of the liquid to be distilled. This speed of evaporation in turn largely determines the quality of the distillate obtained.
It is also known from the prior art to supply the distillation system with auxiliary steam to maintain the amount of steam that is being supplied at a constant level, if the main amount of steam may vary as a result of variations in the operation of a ship's turbine.
Further, it is known if drinking water is to be produced from seawater on a large scale, to employ the waste heat of nuclear reactions which are used in nuclear power plants to produce electrical energy. The steam produced by the heat of the nuclear reactor is supplied to turbines wherein it produces electrical energy, and the waste steam from these turbines, that is the spent steam, is subsequently used for heating a distillation installation which may, for example, operate on the principle of expansive distillation. This approach makes possible an economically feasible production of drinking water from seawater, but at the same time it is subject to substantial difficulties which arise from the fact that distillation systems must be operated with constant amounts of steam. For this reason the steam turbine and distillation system are so constructed in the arrangements using nuclear heat, that both are operated at a predetermined total capacity. When the total energy requirements of the complete arrangement drop below this level, for instance because there is less requirement for electrical energy, then water vapor is directly supplied from the steam boiler to the distillation system, because a reduction in the amount of steam required in distillation systems is as a matter of principle not possible. Further difficulties result from the fact that electrical fluctuations, that is fluctuations in the electrical energy demand, occur quite rapidly and that the amount of steam that is being used in the distillation system cannot be controlled in dependence upon such fluctuations.
Such problems are routinely experienced in all large systems serving for desalinization, production of drinking water from salt water, and similar applications. In fact, in some instances the reasons for the difficulties experienced are not entirely clear. Added to these problems is the fact that the partial pressures in the individual evaporating stages must not deviate too much from one another, because otherwise the phenomenon known as water-hammering will be experienced, which leads to damage to the heat exchangers of the system.
It is also known from the prior art to produce drinking water from salt water by means of solar energy. In this case the aforementioned problems are not experienced, because the water is simply pumped into shallow containers or the like and evaporates from them under the influence of solar heat, to subsequently condense on cooler surfaces. However, this very ancient approach to desalinization and production of drinking water is not suitable for large-scale applications.
All of the aforementioned approaches are suitable for converting contaminated water, such as river water or salt water, into drinking water, water suitable for feeding of boilers, and the like, by means of distillation. It is also known from the prior art to cool a furnace by means of water which is circulated in a closed circuit through a cooling system of the furnace. Heretofore, the steam resulting from this cooling operation was condensed in a surface condenser by means of cooling water or cooling air, and the condensate was returned to the cooling system. However, the heat of condensation is then of course lost, being carried away with the cooling medium.